A power transformer is the second most costly equipment installed in power system. Hence, special care to be taken during installation of a new power transformer in an electrical substation. During accessing the interior of a power transformer, the worker should empty their pockets of all loose articles so that there would not be any risk of falling anything inside the transformer from their pocket.
The tools used during the work, should be securely tied with cotton tap so that they can be recovered if accidentally fall into the transformer interior. Jute should not be used as cleaning materials for a transformer. Loose fibers of the jute may be remained even after completing the cleaning work, which may affect the insulating properties of the equipment in future. Cleaning should be done by clean cotton cloth without any suspended loose fibers. Generally, many parts of a power transformer are transported from manufacturers separately.
All the parts or components dispatched separately should be properly cleaned from inside and outside before being fitted to the transformer body. Special attention, to be given if, interior surface of the any component is found rusty. Transformer oil is also dispatched in separate sealed steel drums. This oil is highly inflammable hence no flame should be brought nearer to the transformer oil.
Transformer oil and the cellulose-based insulating material used in transformer, both absorb moisture easily hence, transformer oil and cellulose-based insulated parts of the transformer should be handled very carefully. The main body of power transformer can be supplied by the manufacturer without or with insulating oil inside it. In both cases it is best practice to circulate dry and warm oil through it until its temperature is 5oC to 10oC above ambient.
This should be done before allowing external access to the interior of the tank. The warm and dry oil should be circulated in the transformer during all the time the transformer remains open to atmosphere. The oil pump and all joints in the oil pipe work should be properly air tight otherwise air will enter in the transformer and get lodged in the vulnerable places such as winding.
Locations and Site Preparation for Installation of a Power Transformer
For installation of a power transformer, no special type of foundation is required. The foundation should be strong enough to carry the heavy load of transformer and it should be efficiently designed to prevent accumulation of water below the transformer. It is essential to drain all the oil coming out from transformer during any fire hazard and hence special oil drainage facilities should be provided in the foundation design.
This last point is strongly recommended especially for large power transformer. The foundation should be created in such a place where easy accessibility can be provided to all indicating and protecting devises.
That means there should be sufficient place all around for proper viewing of diaphragm plates, thermometers, valves, oil gauges, etc. Sufficient and minimum standard electrical clearances of all live parts of the transformer from earth and other live bodies are also to be provided. If the transformer is of Oil Natural Air Natural (ONAN) type, there should be sufficient space available all around the transformer for circulating the air naturally.
If the ONAN transformer is indoor type, the room must be well ventilated. In this case the air inlets and outlets should be of sufficient size and number. Air inlets of the room must be as nearer the floor as possible and air outlets must be as high as the building will allow and this arrangement will facilitate sufficient and efficient circulation of air during natural cooling of transformer. Sometime in case of indoor transformer, exhaust fans are used for forced circulation of air in the room.
Where wheels or rollers are not fitted on the base of the transformer body, level concrete plinth with bearing plates of sufficient size and strength can be adopted for outdoor transformer. To prevent the formation of rust, it is essential to avoid presence of air and water in the space between the plinth and the base of the transformer by use of cretex or similar bituminous compounds.
Where rollers are fitted on the base, suitable rails or tracks should be fitted with the foundation to place and lock the rollers on the rail. Locking of rollers on the rail is essential because it prevents the accidental movement of transformer during earth quakes or any similar hazard.
Preparation of Oil during Installation of Transformer
When transformer is dispatched without oil, required entire oil which is supplied by the manufacturer in separate sealed steel drums. But when the transformer is dispatched with oil in the main tank, the required oil for conservator tank, radiators of transformer, and for other essential parts is dispatched in separate sealed steel drums.
Although the oil supplied by the manufacturer are brand-new but still it is required to be filtered, cleaned and dried before feeling in the transformer. Filtering process must be continued until the transformer oil satisfies the specifications indicated by manufacturer.
The parameters of the oil, to be checked during filter process are generally for electrical strength, resistivity at 27oC, resistivity at 90oC, tan delta value at 90oC, and moisture content in ppm. Oil is easily contaminated hence it is important, when sampling the oil and filling the tank, to keep the oil free from contamination.
All the components used for handling the transformer oil should be cleaned and flushed with fresh transformer oil before use. For collecting sample of the oil for the testing purpose, glass containers with glass stopper is preferable over the metallic container. Cleanliness of the container is essential because very small amount of moisture can affect the test results. Never Use wax for sealing the oil sample containers.
If possible, glass stopper is covered by a pack of silica gel tied in a piece of cloths. Flexible steel hose pipe is preferable for filtering and handling of insulating oil. Some kind of a specially made synthetic rubber pipe or PVC pipe can be used as hose but ordinary rubber hose pipe must not be used in any case as the transformer oil dissolves the sulphur from rubber and is thereby contaminated. The steel hose used for this purpose must also be properly cleaned and free from moisture and loose rust.
Oil Filling in Transformer Tank during Installation
The oil can only be filled in the transformer which satisfies the standard specifications mentioned by the manufacturer of transformer. When the transformer is supplied gas filled, the oil filling is done in the main tank under vacuum.
The large transformers are generally designed to withstand the full vacuum for long periods. During oil filling in the main tank of the transformer, it is preferable to connect the inlet hose pipe at the bottom of the tank. After filling the oil up to the top of the transformer core, maximum suitable vacuum is maintained above the oil level in the tank to minimize the hazard of bubbles lodging in the inaccessible corners of winding.
It should be noted that condenser bushing, diaphragm, Buchholz relay, tap changer board, conservator tank, radiators units etc are not designed for withstanding vacuum hence they should not be subjected to vacuum. That means vacuum processes only to be done in main tank without connecting or with blocking all the above-mentioned components of the transformer.
Drying out Power Transformer using Streamlined Filter Machine
The most common process of drying out a power transformer is by using a streamlined filter machine connected across the inlet and outlet oil hose. The vacuum chamber and oil heater incorporated in the filter machine are mainly utilised for this purpose. Before using the filter machine it should be ensured that the oil previously filled in the machine, must be drained out and the machine is refilled with the transformer oil supplied by the manufacturer.
This oil first circulated inside the machine itself for several times and tested again and again for achieving required result. If the required result is achieved then the machine should be connected across inlets and outlet hose of the transformer and the entire oil should be circulated through the transformer and filter machine several times and the oil sample is to be tested during a specified interval normally after each one hour.
The vacuum pump of the filter machine should have the capacity of creating vacuum as high as possible, but not less than 710 mm of mercury. Where possible, an additional vacuum pump can be connected at the top cover of the tank to keep the oil in the tank under vacuum consistently. This additional vacuum pump may accelerate the drying out process of the transformer.
Although, the oil was filled in the transformer under vacuum from the bottom of the tank, but during drying out process oil is drawn out from the bottom and fed into the top of the transformer tank. This helps to remove any settled moisture/impurities remain in the oil.
The oil circulation in that direction must be continued about 8 to 12 hours and then the cycle is reversed i.e. oil is drawn from the top and fed in the bottom of the tank. During this process the oil must not be heated beyond 85oC in the machine and the temperature of the oil inside the transformer tank should not exceed 75oC.
If this maximum allowable temperature limit is not maintained, the oil may be decomposed due to higher temperature operation. The process of drying out is continued until the satisfactory insulation resistance value is achieved. The insulation resistance or I R value of the oil mainly depends upon its moisture content hence, by analyzing the I R value, the moisture content in the oil can be predicted.
The I R values of the oil taken in a regular interval (at the interval of one hour) are plotted against time. It should be observed from the graph that initially the I R value drops down and this I R value will be low till the moisture is present inside.
Once the entire moisture is removed from the oil, as well as interior of the transformer, the I R value again goes up and becomes steady. Initially insulation resistance value came down because the moisture contained in the core and winding of the transformer were removing in the hot oil thereby moisture content of the oil was increased.
After, this moisture removed from the oil in the filter machine, the insulation resistance value of the oil became again high and steady. The I R values taken at regular intervals as should also be plotted against temperature readings. In the beginning, I R values drop down as the temperature goes up. The I R value will be low till the moisture is present inside.
Once the moisture is removed, the IR value will start rising and then becomes steady. In addition to the drying out process by filter machine, the transformer can also be heated up by short-circuiting the LV winding and supplying a reduced voltage at the high voltage terminals. The value of the applied voltage is so maintained that the current through the short-circuited winding should be under 70% of the rated current.
It must also be of observed during heating up the transformer winding by applying reduced voltage, the temperature of the oil must not exceed 75oC. At the same time temperature of the winding must not exceed 90oC.
Gaskets and Gasket Joints
Transformer gasket are neoprene bonded, cork gasket. The gaskets are supplied loosely by the manufacturer without any bolt holes created on them. The gaskets are usually cut to the size and shape and punched for the holes at site. Joints in the gasket should be located away from the corner and bold holes.
Before using, the gasket must be checked for their proper flexibility and free from local thickening. Neoprene solution is used as adhesive for joints in the gasket. After fitting the gasket the bolts are typed in sequence, a little every time so that uniform pressure is exerted on the gasket until the gasket is compressed to about 2/3 of its original thickness.
Fitting and Fixing Bushings of Transformer
Before fitting the bushings on the transformer, they should be checked for any damage or crack. If they are all right, these should be cleaned by dry cotton cloth. Bushings are normally delivered in complete assembled form. When clamping the bushing to the tank, care should be taken to ensure that an even pressure is obtained to the flange of the porcelain and its gasket by tightening the diagonally opposite nuts alternatively. The power connection at the stud of the bushing should be flexible enough to absorb any vibration. If arcing horns are supplied with the bushing they must be fitted and the gap between the horns must be adjusted as per the drawing supplied with the transformer.
Installation of Conservator Tank
Conservator tank, where fitted should be assembled with its pipe work, etc, making sure that SKATED joints are oil tight and the pipe work is clean and free from moisture. The mechanism of the float type oil gauge inside the conservator is locked to prevent damage during transit. After placing the conservator in position, it should be released by turning the locking bolt in the direction indicated on the plate.
While feeling the conservator with transformer oil, it should be ensured that the oil filled up to the level indicated by the oil gauge on the conservator. On the oil gauge’s dial there is an indication of temperature, this tells the prescribed level of oil in the conservator at that temperature.
For example, if 30oC is indicated in oil gauges dial, the oil should be filled in conservator up to that level when oil temperature is 30oC. But during actual oil filling in conservator tank, we should commensurate with the oil temperature at that time.
Fittings of Silica Gel Breather
Silica gel breather always comes as loose item. The plastic plug sealing on the top of the breather should be removed and fitted to the associated pipe work. The oil cup at the bottom of the breather should be removed, filled with transformer oil to the level indicated on it and then re fitted to the position. Thus the silica gel is prevented from absorbing moisture from atmosphere.
Installation of Explosion Vent
Explosion vent is fitted with suitable gasket. Here care should be taken to ensure that the top diaphragm with its gasket makes an air tight joint. The explosion vent is sent blanket from works and the top blanking plate should not be removed till the oil level inside the transformer comes up of the tank cover.
When a transformer is provided with a Buchholz device, bottom paper diaphragm which acts as a deflector plate, is fitted at the base of the explosion vent to ensure that any gases liberated in the transformer, will pass through the Buchholz relay and not collect in the vent. A hole in the side permits the oil to enter the vent and equalise the pressure on either side of the deflector diaphragm.
In transformers of large rating the space above the oil in the vent is usually connected by a pipe to the top of the conservator to equalise the pressure and vent it to the atmosphere through the breather. For transformer supplied without equalising pipe and air cock is fitted at the top which must be opened when the tank is being filled with the oil and closed again.
Fitting of Buchholz Relay
Buchholz relay is connected across the pipe work between conservator tank and main tank of the transformer. There is generally a Stop Valve between Buchholz relay and conservator tank. The Buchholz relay should be mounted on the pipe work only after the oil level inside the transformer is above the tank cover.
The Buchholz relay is provided with an arrow marked on its housing and this arrow should always point towards the conservator. If the gas release pipe is provided, it must be fitted to the top pocket of the Buchholz relay. In this case the top pockets of Buchholz relay should be opened and gas release pipes should be full of oil during normal operation of transformer. When the gas is to be collected through the gas release pipe, oil will flow out initially followed by the gas.
But when gas release pipe is not provided with Buchholz relay, the top pockets are generally screwed tightly. During fittings of Buchholz relay in the pipe work, the relay should be mechanically locked with the locking screw provided in the relay. It prevents any mechanical damage inside the relay due to transit.
The alarm and trip contacts provided on the relay should be properly identified from the control drawings of the relay before connecting the control wires on those terminals of the relay. The ‘NO’ contacts for alarm and trip should be checked for their proper functioning.
Installation of Temperature Indicator
Oil Temperature Indicator (OTI) and Winding Temperature Indicator (WTI) are fitted in the Marshalling Kiosk. The thermometer bulbs are enclosed in the pockets on the top cover of the transformer. The thermometer pockets for OTI and WTI should be filled with transformer oil during inserting the thermometer bulbs inside them.
Both Oil Temperature Indicator and Winding Temperature Indicator are provided with maximum pointers and mercury switches. The steel capillary tubes are normally quite flexible but it should not be bent sharply and repeatedly and should be supported by clips to prevent unnecessary sagging.
Before installation, the instrument should be checked for proper calibration by hot and cold oil bath. The mercury switches are adjusted to make contact at the desired temperature depending upon the site condition i.e. ambient temperature, loading condition, etc. The Current Transformers connection of the WTI is made to the thermometer pocket as per instructions given on the WTI terminal board.
Installation of Cooling Equipment
The radiator units are washed by clean dry oil before fittings to the main body of the transformer. After fittings the radiators with proper gasket work, are filled with transformer oil. The air should be released by loosing air release pocket at the top of the header and also at each radiator unit.
When only the drying out process of main tank is completed, the interconnecting valves between the tank and the radiator unit are opened. This is because radiator unit should not be subjected to the vacuum, applied in the main tank during drying out process. The cooling fans and oil circulating pumps should also be fitted and proper wiring should be done according to the cooler control drawings provided with the transformer.
Fitting of On Load Tap Changer
The tap changer, if dispatched separately, it is to be fitted onto the tank, The insulation resistance value of each tap changer lead should be measured and the cause of low values, if any, investigated.
The lead from the tap changer should then be connected to their respective position on the terminal board provided on the tank. The tightness of all connections of the selector switch and terminal board should be ensured.
Sometimes it is difficult to get good insulation resistance values if there is moisture in the tap changer board or on any of the parts of the selector switch and diverter switch. Then it should be ensured that selector switch and diverter switch are filled with clean dry processed oil.
To remove the moisture from the terminal board, it is preferable to circulate the transformer oil in the selector switch tank and main transformer tank simultaneously and at as high a temperature as permissible. If some other special instructions are provided, the same should be followed.
Fitting of Off Load Tap Changer
If the transformer is supplied with off load tap changer, it is previously assembled in the transformer itself. No extra connection to be done unlike in the case of on load tap changer. In this case only precaution is maintained and that transformer should be isolated from the system during changing of tap from one position to other.
Earthing of Transformer
The main tank of the transformer should be effectively earthed at least more than one point. This is to be noted that the earthing connection should be of sufficient cross-section to carry the fault current for at least 30 seconds. The fans, oil pumps, neutrals, marshalling kiosk etc. should also be properly earthed.
Completion of Erection Work for Installation of Power Transformer
Any other work such as wiring of various alarm/trip contacts from various accessories of the transformer to marshalling kiosk, settings of relays, checking of protection scheme, etc. should be completed before the transformer is cleared for commissioning.